media: pulse8-cec/rainshadow-cec: make adapter name unique

The CEC adapter name used by the pulse8-cec and rainshadow-cec USB device drivers
was a fixed string, but it should be unique if you connect multiple of these devices
to the same computer.

Use dev_name(&serio->dev) instead, which make it unique again.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>